This application is a continuation-in-part of co-pending patent application Ser. No. 452,603 filed on Dec. 23, 1982 and now abandoned.
This invention relates to acousto-optic filters and, more particularly, to an electrically tunable acousto-optic filter assembly which utilizes the noncollinear interaction of light (i.e., polarized light in this instance) and acoustic beam in an acoustically anisotropic, optically birefringent, medium to attain improved spectral resoluton and increased aperture of the filter assembly.
It is to be remembered that in a noncollinear filter the light (i.e., optical) beams within the acoustically anisotropic, optically birefringent medium (such as in a birefringent, crystal, and hereinafter referred to as such) are noncollinear with the acoustic (i.e., sound) beam. The most significant feature of the noncollinear acousto-optic filter is that a narrow filter bandpass can be maintained for a relatively large cone of incident light. This large angular aperture characteristic is due to the proper choice of acousto-optic geometry, wherein the tangents to the locus of the incident and diffracted light wavevectors are parallel. Hereinafter, "tunable acousto-optic filter" will be defined as the type of optical filter that operates on the basis of acousto-optic diffraction satisfying the "parallel tangents" condition. This type of nonlinear tunable acousto-optic filter is described by the present inventor on pages 370-372 of the October 1974 issue of the Applied Physics Letters, Vol. 25, No. 7 and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Acousto-Optic Filter." Such tunable acousto-optic filters are clearly distinguishable from the type with small angular aperture as described in U.S. Pat. Nos. 3,944,334, 3,944,335 and 3,953,107, all of which rae entitled "Acousto-Optic Filters."
Additionally, it is to be remembered that the more restrictive term "transverse configuration" is intended to mean herein, as it does it the art, that the acoustic (sound) beam is perpendicular to the light (optical) beams. More specifically, and more accurately, it is the energy flow direction (i.e., the group velocity) of the acoustic wave which is perpendicular to the light beams and, of course, vice-versa. In this regard more detailed technical information is readily available in U.S. Pat. No. 4,342,502, entitled "Transverse Tunable Acousto-Optic Filter".
One of the major difficulties in the practical implementation of tunable acousto-optic filters (hereinafter referred to as TAOF) is the limited aperture available. The optical aperture of an acousto-optic filter is limited by the drive power. This is particularly serious in the infrared region, since the drive power required is proportional to the square of optical wavelength. One filter configuration that provides increased optical area is the transverse configuration (hereinbefore previously referred to, and hereinafter to be described in detail) in which telluride oxide (TeO.sub.2) is used as the birefringent crystal. First, telluride oxide has an extremely large figure of merit that results in reduced power need; and, secondly, the transverse configuration allows an extended acoustic path, and thereby can be used to achieve larger aperture without increase of drive power.
However, the usual geometry of a transverse configuration telluride oxide TAOF is not suitable for realizing high spectral resolution. In many applications, such as laser detection and identification, high spectral resolution is one of the most important requirements.
The basic difficulty of the usual geometry of a transverse configuration telluride oxide TAOF is that the minimum transverse dimension (hereinafter referred to as the "dead space") along the optical aperture is about one and one-half times that of the interaction length. This means that for high spectral resolution, the "dead space" (as well as the required crystal size) will be too large for practical purposes.
Furthermore, in the usual configuration of the transverse TAOF, a large transducer must be used to achieve high spectral resolution. The use of a large size transducer will make the impedance so low that it greatly increases the technical difficulty of providing adequate impedance matching.
Accordingly, it is fair and accurate to state that what is needed in the art and is not available is a tunable acousto-optic filter with improved spectral resolution and increased aperture.